1. Technical Field
A method and system for fluidization of particles, particularly agglomerates of nanoparticles and/or nanopowders, are provided wherein a fluidizing medium (e.g., a fluidizing gas) is directed in a first direction and an opposite jet flow is introduced to the chamber. The opposite jet flow is effective in enhancing the fluidization behavior of the disclosed system, even if the opposite flow is reduced and/or discontinued at a point in time after desired fluidization parameters are achieved. In fluidization systems including agglomerates of nanoparticles, the jet flow need not necessarily be opposite to the flow of the fluidization medium to provide enhanced results, although if fluidization of all the powder contained in the chamber is desired, oppositely directed jet flow is required.
2. Background Art
Challenges are frequently encountered in fluidizing systems, particularly systems that include small particles. Indeed, the small size and large surface area of nanoparticles and nanopowders increase the cohesive forces, such as van der Waals forces, acting on and between individual nanoparticles and nanoagglomerates. Due to these interparticle forces, agglomerates of various sizes and shapes are frequently formed in fluidization chambers. The presence of such agglomerates significantly limits the efficacy of conventional fluidization techniques with respect to nanoparticle and/or nanopowder systems.
Based on the Geldart classification system, powders having particle sizes less than about 20-30 microns (hereinafter μm) are defined as Geldart Group C powders. Geldart Group C powders are also referred to as fine cohesive powders. Nanoparticles are generally defined as particles having dimensions on the scale of nanometers. In most instances, nanoparticles are defined as having dimensions less than about 100 nm. Interest in the area of nanoparticle fluidization has increased due to increasing and potential uses for nanoparticles.
Many methods of enhancing fluidization by disrupting forces between particles are discussed in the literature. Lu et al. separates these fluidization aids for Geldart Group C particles into external methods (i.e., methods that overcome forces between particles using an external force) and intrinsic methods (i.e., methods that decrease forces between particles by changing conditions proximate the particles). [Lu, Xuesong, Hongzhong Li, “Fluidization of CaCO3”] Fluidization aids include flow conditioners, mechanical vibration, sound-assisted fluidization, fluidization with magnetic/electric fields, pulsed fluidization and centrifugal fluidization. [Yang, Wen-Ching. “Fluidization of Fine Cohesive Powders and Nanoparticles—A Review,” Journal of the Chinese Institute of Chemical Engineers, 36(1), 1, (2005).] Flow conditioners may include additives, for example, an anti-static surfactant. [Hakim, L. F., J. L. Portman, M. D. Casper, A. W. Weimer, “Aggregation Behavior of Nanoparticles in Fluidized Beds,” Powder Technology, 160, 153, (2005).]
U.S. Patent Application 2006/0086834 by Pfeffer et al. teaches “coupling the flow of a fluidizing gas with one or more external forces, the combined effect is advantageously sufficient to reliably and effectively fluidize a chamber or bed of nanosized powders.” [U.S. Patent Application 2006/0086834 at [0024]]. Pfeffer et al. describe external forces to include: “magnetic, acoustic, centrifugal/rotational and/or vibration excitation forces.” [See, also, Yang, Wen-Ching. “Fluidization of Fine Cohesive Powders and Nanoparticles—A Review,” Journal of the Chinese Institute of Chemical Engineers, 36(1), X, (2005).]
Sound assisted fluidization is outlined by Zhu et al. as a method for enhancing fluidization. [Zhu, Chao, Guangliang Liu, Qun Yu, Robert Pfeffer, Rajesh N. Dave, Caroline H. Nam, “Sound assisted fluidization of nanoparticle agglomerates,” Powder Technology, 141, 119 (2004).] Further, Martens describes reducing the average size of particles or agglomerates suspended in a fluid by combining with a second fluid which includes a metallic compound and flowing the combined fluid through one or more magnetic fields. [U.S. Patent Application 2005/0127214 to Martens, published on Jun. 16, 2005.]
Alfredson and Doig describe a method for increasing fluidization of particles having diameters of less than about 50 μm by using fluidizing pulses. [Alfredson, P. G., I. D. Doig. “A Study of Pulsed Fluidization of Fine Powders,” Chemeca '70, 117, (1970).] According to Alfredson et al., providing the fluidizing medium in a series of pulses was shown to overcome channeling and poor gas-solids contact for fine particles.
Studies at Monash University by Akhavan and Rhodes analyzed pulsed fluidization of cohesive powders which involved varying the velocity of the fluidization medium as a function of time. [http://users.monash.edu.au/˜rhodes/projects.htm#2 on Aug. 22, 2006.] The studies of Akhavan et al. suggest oscillating a portion of the fluid flow by supplying a constant flow and a pulsed flow into a windbox of a fluidized bed. Akhavan suggests that “this new bed structure can be sustained for a considerable period of time after the pulsation is stopped.” [http://www.monash.edu.au/chemeng.seminars/akhavan%20—25may-06.pdf#search=%22ali%20akhavan%2C%20pulsed%22 on Aug. 22, 2006.]
U.S. Pat. No. 6,685,886 to Bisgrove et al. teaches using a fluidization supply system in combination with an agitation system and a spray gun to supply a fluid via a duct to particles resting on a screen. Bisgrove et al. disclose spray guns configured to force particles back down into the expansion chamber to foster growth of the particles. Bisgrove et al. state that “spray gun 74 continues to spray solution until the particles P have been enlarged to the desired size from coatings or agglomeration. At that point, the spray gun 74 is turned off . . . the agitation system 12 continues to agitate the particles P in the bed 22 of the product chamber 14 to prevent undesired agglomerations from occurring.”
U.S. Pat. No. 4,007,969 to Aubin et al. discloses a device for fluidizing and distributing a powder in a gas suspension. Aubin et al. disclose that “the pressurized gas, carrying a powder made of a mixture of particles, grains and agglomerates, is fed from a distributing means (not shown), located upstream of inlet conduit 10.” Aubin et al. further disclose that “[t]his fluidization step results both from the interaction of the two carrier-gas jets at the extremity of nozzles 22, 24, and from the spherical shape of chamber 20.” [Col. 2, lines 35-39.] Aubin et al. state that their disclosed system is capable of “extending its range of use to very fine powders, of a grain-size of about 1 micron or less.”
In U.S. Patent Application 2005/0127214, Marten et al. disclose a method for reducing the average size of metallic compound particles or agglomerates suspended in a fluid. The system of Marten et al. involves flowing a fluid with metallic compound particles or agglomerates suspended through a magnetic field to reduce the average size of a substantial portion of the metallic compound particles or agglomerates by at least 25%.
In U.S. Patent Application 2005/0274833, Yadav, et al. disclose a system for reducing agglomerates to particles through “shear forces, or other type of stress,” e.g., “a ball mill, or jet mill, or other types of mill, or sonication, or impaction of particles on some surface.” Yadav et al. further disclose using an elevated temperature, in combination with a catalyst, such as a solvent, to reduce agglomerate size.
U.S. Pat. No. 4,261,521 to Ashbrook describes a method for reducing molecular agglomerate size in fluids. Two vortex nozzles are positioned opposite each other and fluid flow from the nozzles is controlled so that the fluid from one nozzle rotates in an opposite direction to fluid emerging from a second nozzle. The fluid streams collide and the collision reduces agglomerate size.
U.S. Pat. No. 4,095,960 to Schuhmann, Jr. discloses a process and apparatus for converting particulate carbonaceous fuel, such as high-sulfur bituminous coal, into a combustible gas. An ignited fluidized bed of the particulate carbonaceous fuel is formed in a closed-bottom shaft furnace and a jet stream of oxygen is directed downward into the bottom zone by means of an oxygen lance passing axially through a roof enclosure. The oxygen stream forms a dynamic, highly turbulent suspension of particulate fuel. Particulate reaction products travel in a toroidal manner in the bottom zone of the fluidized bed, continuously removing effluent gases formed by reaction of oxygen with the fluidized bed, and maintaining the fluidized bed by continually feeding makeup fuel to the shaft furnace. In a bench-scale reactor, a very small orifice (approximately 0.025 inch in diameter) is drilled into the nozzle end of the lance.
U.S. Pat. No. 5,133,504 to Smith et al. discloses a fluidized bed jet mill that includes a grinding chamber with a peripheral wall, a base, and a central axis. An impact target is mounted within the grinding chamber and centered on the chamber's central axis. Multiple sources of high velocity gas are mounted in the peripheral wall of the grinding chamber, are arrayed symmetrically about the central axis, and are oriented to direct high velocity gas along an axis intersecting the center of the impact target. Alternatively, the sources of high velocity gas are oriented to direct high velocity gas along an axis intersecting the central axis of the grinding chamber. Each of the gas sources has a nozzle holder, a nozzle mounted in one end of the holder oriented toward the grinding region, and an annular accelerator tube mounted concentrically about the nozzle holder. The accelerator tube and the nozzle holder define between them an annular opening through which particulate material in the grinding chamber can enter and be entrained with the flow of gas from the nozzle and accelerated within the accelerator tube to be discharged toward the central axis. In a disclosed embodiment, an Alpine model AFG 100 mill with three nozzles is disclosed, each nozzle having an inside diameter of approximately 4 mm and an outer diameter of about 1.5 inches.
U.S. Pat. No. 6,942,170 to Casalmir et al. discloses a jet mill that includes plural nozzle devices for discharging a composite stream of high velocity fluid. Each nozzle device includes a plural odd number of nozzle openings for discharging an individual stream of high velocity fluid. In a disclosed embodiment, five (5) PONBLO nozzle devices having nozzle size of 15 mm were utilized.
In a publication entitled “Fluidization of Fine Powders in Fluidized Beds with an Upward or a Downward Air Jet,” the authors describe a study directed to the hydrodynamic behavior of fine powders in jet-fluidized beds. [R. Hong, J. Ding and H. Li, “Fluidization of Fine Powders in Fluidized Beds with an Upward or a Downward Air Jet,” China Particuology, Vol. 3, No. 3, pages 181-186, 2005.] As stated by Hong et al. at page 181:                Study on a fluidized bed with a downward jet is both theoretically and practically important. Shen et al. (1990a; 1990b) studied experimentally a downward gas jet in a two-dimensional fluidized bed. Werther and Xi (1993) investigated the jet attrition of catalyst particles in a gas-fluidized bed with a downward jet.In the Shen et al. study referenced above, the jet nozzle velocity was 51 to 124 m/s and the nozzle diameter was 6 mm. In the Werther and Xi investigation referenced above, the nozzle size was 0.5 and 2 mm and the nozzle velocity was 100 m/s. As further stated in the Hong et al. publication at page 181, “[g]as jets with high speed were used to break up the agglomerates of cohesive powders to improve fluidization quality. A downward jet was used instead of an upward jet in order to avoid jet penetrating through the entire bed.” The experimental work and technical discussion provided by Hong et al. is limited to the fluidization of Geldart type A FCC particles, and cohesive glass beads of mean size of 40 μm and the use of relatively large nozzles for the generation of the jets.        
Despite efforts to date, a need remains for effective, reliable and cost effective systems and methods for fluidizing particle and powder systems that are resistant to fluidization, e.g., based on high inter-particle forces. In particular, a need remains for effective, reliable and cost effective systems and methods for fluidizing beds that include nanoparticles and/or nanopowders. These and other needs are satisfied by the systems and methods disclosed herein.